This invention relates in general to scatterometers and in particular, to spectroscopic scatterometric systems and methods employing models for measuring parameters of a diffracting structure as well as related applications to sample processing.
As the integration and speed of microelectronic devices increase, circuit structures continue to shrink in dimension size and to improve in terms of profile edge sharpness. The state-of-the-art devices require a considerable number of process steps. It is becoming increasingly important to have an accurate measurement of submicron linewidth and quantitative description of the profile of the etched structures on a pattern wafer at each process step. Furthermore, there is a growing need for wafer process monitoring and close-loop control such as focus-exposure control in photolithography.
Diffraction-based analysis techniques such as scatterometry are especially well suited for microelectronics metrology applications because they are nondestructive, sufficiently accurate, repeatable, rapid, simple and inexpensive relative to critical dimension-scanning electron microscopy (CD-SEM).
Scatterometry is the angle-resolved measurement and characterization of light scattered from a structure. For structures that are periodic, incident light is scattered or diffracted into different orders. The angular location θr of the mth diffraction order with respect to the angle of incidence θi is specified by the grating equation:
                                          sin            ⁢                                                  ⁢                          θ              1                                +                      sin            ⁢                                                  ⁢                          θ              r                                      =                  m          ⁢                      λ            d                                              (        1        )            where 8 is the wavelength of incident light and d the period of the diffracting structure. Spectral scatterometry performs the above measurement using a variety of transmitted light that can be used for measurement of the grating parameters.
The diffracted light pattern or spectrum from a structure can be used as a “fingerprint” r “signature” for identifying the dimensions of the structure itself. In addition to period, more specific dimensions, such as width or critical dimension (CD), step height (H), and the shape of the line, and angle of the side-walls (SWA), or other variables referred to below as parameters of the structure, can also be measured by analyzing the scatter pattern.
In scatterometry, a diffraction model of the diffracting structure or grating is first constructed. Different grating parameters outlined above are parameterized and the parameter space is defined by allowing each parameter to vary over a certain range. A look-up-table is then constructed offline prior to measurements. The look-up-tables, also-called libraries, are multi-dimensional with the parameters such as CD, height and wall angle as the variable of each dimension. The tables contain typically, a collection of spectra where each spectrum is a plot of a measured diffraction reflectance or transmittance versus wavelength or illumination angle corresponding to a particular set of values of the parameters. After the sample spectrum is measured, it is compared to all the spectra in the look-up-table to find the best match and the value or values of the one or more parameters are then determined by the values at which the best match is found.
The look-up-tables are multi-dimensional and need to cover a number of parameters extending over different ranges. The end result is a multi-dimensional sampling grid with each point on the grid being a spectrum that contains hundreds of data points. Such tables are extremely time consuming to calculate and difficult to refine. If any parameter during real time measurement falls outside the sampling grid, or any dependent variables are different from what have been used for constructing the look-up-table, then the tables become useless and have to be reconstructed, which may take days. This drawback significantly reduces the value of integrated CD measurement systems, of which the main goal is to reduce the time delay from process to metrology results.
It is, therefore, desirable to provide an improved technique for deriving the important parameters of the diffracting structure from the measured data.